This invention relates generally to the detection of conductors near inductive coupling loops, and more specifically to the detection of cookware on a cooktop or heating surface and determination of the size and/or the temperature of the cookware located thereon.
Cookware detection, size determination, and temperature determination features for induction heating cooking appliances provide added convenience to usera as well as allowing an added level of safety to be incorporated into cooking surface control schemes. For example, in some induction cooktops, power is disabled to heating elements until a presence of an item of cookware is detected on the cooktop, therfore providing automatic power control to heating elements while conserving energy and at least partially reducing a likelihood of injury from contact with unoccupied heating elements.
In at least one known pan detection system, an induction coil is used and the presence of an item of cookware on the cooking surface induces frequency changes detected in a detection circuit coupled to the controls for operating heating elements. However, electronic components used in such applications are sensitive to heat degradation because the heat generated in a cooking surface environment changes electronic components values and tolerances over time. In addition, during cooking cycles, the electronic components are exposed to fluctuating heat generated in the cooking elements so electronic component values and characteristics fluctuate during the cooking cycle. As applied to the known detection systems, the heat fluctuation in the cooking environment, and thus the fluctuations in component value, induces fluctuations in a detected frequency in the detection circuit. These fluctuations in frequency require a sophisticated control circuit with a particular bandwidth to accurately detect presence of cookware and to control heating elements accordingly.
Accordingly, it would therefore be desirable to provide a circuit for accurately detecting cookware presence and size for an induction based cooking system that is not based upon frequency change as the detection mechanism, thereby eliminating the problems associated with fluctuating component values induced by exposure to heat in the cooking environment.
In an exemplary embodiment of the invention, a method of detecting the presence of a conductor, for example a piece of cookware, near an inductive coupling loop includes the steps of introducing an excitation signal to a coupling loop, measuring the amount of signal distortion induced in the coupling loop by the excitation signal, and monitoring the harmonic content of the distorted signal.
The inductive coupling loop is located within a cooking surface or cooktop, near a heating element. A signal generating circuit is used to generate an excitation signal that is applied to the coupling loop. The effect of a conductor, such as a piece of cookware, is to distort the excitation signal as the cookware is positioned over the coupling loop. The distortion is in the form of harmonics of the excitation signal, and the harmonics are detected with a detection circuit. Therefore by detecting signal distortion harmonics rather than frequency changes, a cookware detection system is provided that is not as susceptible to heat effects of the cooktop in comparison to known detection systems.